Retrieves embedded metadata timestamps from Kafka messages.
If a record has a negative (invalid) timestamp value the timestamp is returned as-is;
in addition, a WARN message is logged in your application.
Returning the timestamp as-is results in dropping the record, i.e., the record will not be processed.

Embedded metadata timestamp was introduced in "KIP-32: Add timestamps to Kafka message" for the new
0.10+ Kafka message format.

Here, "embedded metadata" refers to the fact that compatible Kafka producer clients automatically and
transparently embed such timestamps into message metadata they send to Kafka, which can then be retrieved
via this timestamp extractor.